1. Field of the Invention
This invention relates to an ultrasonic imaging atherectomy apparatus for imaging and removing atherosclerotic plaque burden in human arterial and peripheral vasculature, and more particularly to improvements in imaging component form, connection, positioning and control for reducing cost, patient stress and the risk of induced emboli.
2. Previous Art
Atherosclerosis is a well known condition characterized by fatty and/or calcified deposits in the coronary arteries and peripheral blood vessels of the human body. These deposits are referred to generally as atheroma. There are many manifestations of atherosclerosis, including angina, hypertension, myocardial infarction, strokes, and the like.
Initially the atheroma deposit on the walls of blood vessels as soft, flexible accumulations. With time, the soft atherosclerotic material may become a calcified and hardened plaque. Stenoses generally refers to areas of blood vessel which are blocked by atheroma, plaque, or other material, the blocking material being referred to as plaque burden or stenotic material.
Several procedures have been developed for treating stenoses, such as balloon angioplasty where the stenoses are partially or completely compressed by a balloon catheter inserted into the blood vessel and inflated adjacent to a stenosis whereby the blood vessel becomes more open. An example of such a procedure and its apparatus is provided in Mueller et al, U.S. Pat. No. 5,041,089. Mueller teaches a catheter including a tubular structure having inner and outer walls. The outer wall is enlarged at a distal end of the catheter to define an annular lumen used to inflate the wall as a balloon. In this way the stenoses is compressed. However, this procedure is susceptible to re-occlusion of the vessel at a later time because the stenotic material is not removed.
A number of atherectomy procedures have been developed using various catheter instruments for surgically removing portions of the stenosis. Examples of such procedures are provided in U.S. Pat. Nos. to Gifford et al., No. Re. 33,569, Gifford, No. 5,071,425, and Simpson et al., No. 5,092,873, each of which is incorporated herein by reference. These patents generally disclose a housing having a window, a cutter enclosed within the housing and exposed through the window for removing a portion of the stenosis, a balloon used for urging the cutter against the stenotic material to be removed, a lumen for inflating and deflating the balloon, a housing and a flexible elongated catheter body connecting the housing and body.
Experience has shown such procedures to be valuable despite a necessity for repeated insertion and removal of different instruments as these are found to be better or worse suited for the situation arising during treatment. The repeated cycles of insertion and removal result in prolonged treatment time with an attendant cost and heightened risk of injury to the patient due to ischemia or damage to blood vessel walls.
Ultrasonic imaging has been used to improve identification of the nature, extent and location of the stenoses during surgery. Ultrasonic imaging has also been used to determine the method of treatment and the resultant effect. The use of such imaging has successfully reduced both treatment time and patient risk. Examples of the technique are presented in Yock, U.S. Pat. Nos. 5,000,185 and 4,794,931. As taught in these patents, ultrasonic energy is generated by a transducer located at a distal end of, or within a vascular catheter. The transducer is manipulated to sweep an ultrasonic signal in a desired pattern. Ultrasonic energy reflected from the interior surface of the blood vessel, including any stenosis or occlusion present, is processed and the result used to display an image or profile of the interior of the vessel.
The difficulties of design, manufacture and use of ultrasonic devices for ultrasonic imaging have been numerous. The size of material, typically less than 1 mm outside diameter and high frequency, typically greater than 20 Mhz, have made the selection of the materials quite important. In general, PZT ceramic is preferred because it converts energy, mechanical and electrical, more efficiently than other materials. However, PZT ceramic material lends itself to a phenomenon commonly referred to as "ringing" when not constructed to precise standards. Ringing is defined as the energy emitted by the transducer after the cessation of the transmitting signal. As will be appreciated, if the ringing continues it will mask the echo from near by tissue making the device unusable. PVDF films have also been used and while they do not typically exhibit ringing, such materials are not efficient at converting electrical energy to mechanical energy. However, such materials are efficient at converting mechanical energy to electrical energy. Additionally, such materials have high electrical output impedance making such materials unsuitable for driving a transmission lines directly.
In catheters which contain guiding means, the guide means take the form of cylindrical members, or flexible wires interior to the catheter. Both these structures, being generally centrally located in the catheter, increase the overall cross-section of the catheter, thus limiting its use in smaller blood vessels.
In those devices which combine the cutting and imaging functions in a single device, the physical proximity of the transducer and the cutting edge can result in unintended and unavoidable removal of healthy tissue as the combined transducer/cutter is manipulated during the scanning phase. The risk of induced emboli is thus increased.
What is needed is an improvement in transducer configurations which will allow better imaging capability at smaller catheter dimensions.
There is need also for a flexible guide means which will reduce the overall cross-section of catheters thereby allowing their use in smaller blood vessels.
Finally, there is a need for isolating the movement of the cutting edge from exposure to the area being analyzed during the process of moving or directing the ultrasonic energy for image scanning.